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A comparison of the convective and diffusive flux of organic contaminants through landfill liner systems

Contribution of the Texas Agricultural Experiment Station, Texas A&M University System, College Station, Texas, USA

J.C. Thomas

Soil & Crop Sciences Department, Texas A&M University, College Station, Texas, USA

In an effort to protect groundwater, waste disposal regulations now require the use of composite liners consisting of com pacted soil overlain with a flexible plastic membrane liner in both hazardous and municipal solid waste landfills in the USA. When properly installed, composite liners decrease the convective flux of leachate flowing through flaws in the plas tic ; however, even if all flaws could be eliminated, organic chemicals will diffuse through the liner system. Although actual field measurements are not available on the flux of contaminants through landfill liners, field measurements of leachate concentrations at some landfills, leak rates at other landfills and laboratory data on diffusion coefficients, allow estimation of the flux so that convective and diffusive trans port can be calculated. Calculations were made to estimate the flux of five organic chemicals through liners of various designs. For the flexible membrane liner alone, the flux of organic chemicals is dominated by diffusion, and transport by convective flux through flaws is one or two orders of magni tude less. For a 100 cm thick clay liner alone, the flux of organic chemicals is dominated by convective flow since the long path length limits diffusive flux. For a composite liner including a flexible membrane over a 100 cm thick clay liner, the flux of organic chemicals is dominated by convective transport since the diffusion path length through the compos ite liner is much greater than that through the flexible mem brane liner (FML) alone. For a typical composite liner in which the FML contains 30 holes/ha ^-1, the convective flux of organic chemicals is typically two orders of magnitude greater than the diffusive flux. A liner consisting of an FML, or an FML combined with a thin layer of low permeability soil, will allow a much greater flux of contaminants than a composite liner consisting of an FML and a thick layer of low permeabil ity soil.

Key Words: Diffusion • flux calculations • composite liner • landfill leachate • groundwater

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